In antenna systems it is often desirable to reduce the physical size of the radiating structure. A typical example is a microstrip patch antenna. There are well-known engineering trade-offs between the physical height of the antenna and its return loss bandwidth. One technical approach to improve this engineering trade is to employ an artificial magnetic conductor (AMC) such as the high-impedance surface of Sievenpiper et al. in “High-Impedance Electromagnetic Surfaces with a Forbidden Frequency Band,” IEEE Trans. on Microwave Theory and Techniques, Vol. 47, No. 11, November 1999, pp. 2059-2074. Also see U.S. Pat. No. 6,262,495 entitled “Circuit and method for eliminating surface currents on metals.” Planar antenna elements may be placed in close proximity to an AMC surface, as close as 1/1000 of a free space wavelength and radiate efficiently over relatively narrow frequency bands.
A significant issue with AMC-type antennas is the parasitic excitation of TM modes by the feed network. The wire probes or vias are typically routed vertically from the RF backplane through the AMC substrate to feed an antenna element located next to the front exterior surface of the AMC. These wire probes have an interaction (that is, a coupling) with any vertical electric field inside the AMC substrate, including those fields associated with TM surface wave modes. Even though the bound TM surface wave mode is cutoff, evanescent TM modes will still be excited by the wire probes. Such evanescent TM modes are parasitic modes which store unwanted energy, raising the antenna Q factor, and are manifest as resonances in the antenna's return loss. Center-fed elements which require balanced feeds may be excited on an AMC surface by routing a two-wire line vertically through the AMC substrate. See, for example, Azad et al. in “Novel Wideband Directional Dipole Antenna on a Mushroom Like EBG Structure,” IEEE Trans. On Antennas and Propagation, Vol. 56, No. 5, May 2008, pp 1242-1250. However, capacitive discontinuities are present on this balanced feedline at the plane where the feedline passes through the RF backplane of the AMC, and at any plane where the feedline passes through a capacitive patch or between two adjacent capacitive patches at the front exterior surface of the AMC. The capacitive discontinuities limit the achievable return loss bandwidth.